This invention relates to a core-sheath conjugated yarn, and more specifically, it relates to such yarn as above, wherein the core comprises a polyolefin and the sheath comprises a crystallizing resin other than polyolefin. The invention further relates to a process for the manufacture of the conjugated yarn of the above kind which presents a favorable strength characteristic.
It is well known to produce a sheath-and-core type conjugated yarn in such a way that mutually exclusive synthetic resins are extruded from a concentric composite orifice unit and the extruded product is then subjected to a stretching step. However, it should be noted that different spinnable synthetic resins have respective and different stretching conditions adapted for the realization of their respective optimum strength characteristics. Thus, the strength of a yarn element contained in the conjugate yarn is naturally and highly dependent upon the conditions governing the after-stretching step. If the composite yarn element has been subjected to different after-stretching conditions than those which are optimum, it will result in a rather inferior strength characteristic. As an example, a polyolefinic resin, such as polyethylene or polypropylene, requires generally a rather higher stretching ratio than other crystalline resins for optimum strength characteristic. The ratio is preferably 7-10 for polypropylene yarns and the tensile as well as knot strength thereof will substantially drop when the extruded yarns are stretched with a lesser stretching ratio. On the other hand, the optimum stretching ratio for extruded polyamide or polyester, as a further example, amounts generally to 4.0-6.5. When this kind of yarn or monofilament is forcibly stretched with a higher ratio than above specified, it could be experienced that the yarn is at least fibrillated or upon occasion even subject to breakage. It is, therefore a common practice as adopted by and among those skilled in the art to stretch an unoriented composite and conjugated yarn of polyolefin/polyamide or polyolefin/polyester, to a degree of stretch ratio not higher than the maximum allowable one for polyamide or polyester, although, in this case, the polyolefin component is not stretched enough to present its sufficient strength value.
Fishing lines are required to have highest possible yarn stretch for a predetermined yarn diameter or denier. However, fishing lines when they are manufactured according to the known conjugated process hereinabove described, leave much to be desired in their strength requirements.